Date: Thu, 15 Mar 2018 15:16:51 -0700 From: Kees Cook <keescook@...omium.org> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Andrew Morton <akpm@...ux-foundation.org>, Josh Poimboeuf <jpoimboe@...hat.com>, Rasmus Villemoes <linux@...musvillemoes.dk>, Randy Dunlap <rdunlap@...radead.org>, Miguel Ojeda <miguel.ojeda.sandonis@...il.com>, Ingo Molnar <mingo@...nel.org>, David Laight <David.Laight@...lab.com>, Ian Abbott <abbotti@....co.uk>, linux-input <linux-input@...r.kernel.org>, linux-btrfs <linux-btrfs@...r.kernel.org>, Network Development <netdev@...r.kernel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com> Subject: Re: [PATCH v4 1/2] kernel.h: Introduce const_max() for VLA removal On Thu, Mar 15, 2018 at 2:42 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote: > On Thu, Mar 15, 2018 at 12:47 PM, Kees Cook <keescook@...omium.org> wrote: >> >> To gain the ability to compare differing types, the arguments are >> explicitly cast to size_t. > > Ugh, I really hate this. > > It silently does insane things if you do > > const_max(-1,6) > > and there is nothing in the name that implies that you can't use > negative constants. Yeah, I didn't like that effect either. I was seeing this: ./include/linux/kernel.h:836:14: warning: comparison between ‘enum <anonymous>’ and ‘enum <anonymous>’ [-Wenum-compare] (x) > (y) ? \ ^ ./include/linux/kernel.h:838:7: note: in definition of macro ‘const_max’ (y), \ ^ net/ipv6/proc.c:34:11: note: in expansion of macro ‘const_max’ const_max(IPSTATS_MIB_MAX, ICMP_MIB_MAX)) ^~~~~~~~~ But it turns out that just doing a typeof() fixes this, and there's no need for the hard cast to size_t: size_t __error_not_const_arg(void) \ __compiletime_error("const_max() used with non-compile-time constant arg"); #define const_max(x, y) \ __builtin_choose_expr(__builtin_constant_p(x) && \ __builtin_constant_p(y), \ (typeof(x))(x) > (typeof(y))(y) ? \ (x) : (y), \ __error_not_const_arg()) Is typeof() forcing enums to int? Regardless, I'll put this through larger testing. How does that look? -Kees -- Kees Cook Pixel Security
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.